A typical example of the electronic musical instrument is disclosed in Japanese Utility Model Publication (Kokoku) No. 59-13657 and is provided with a tone-sustaining controller which is operative to control the decay time period of a tone produced by a sound unit on the basis of a tone signal produced by a tone generator. Namely, upon depression of a key of the keyboard, a key state signal representative of the key operation is produced by a key switch circuit associated with the keyboard and is temporally stored in a latch circuit synchronous with an actuation of a tone-sustaining switch. In other words, the latch circuit is clocked by a timing signal produced upon actuation of the tone-sustaining switch for the synchronous latching operation. The key state signal stored in the latch circuit is ANDed with a switch-state signal representative of the actuated state of the tone-sustaining switch to produce an output signal indicative of allowing the tone to be elongated after key releasement. In so far as the key is depressed, the sound unit causes the intensity of the tone to gradually decay, and, upon key releasement during the decay time period, the sound unit rapidly terminates the production of the tone under inactivation of the tone-sustaining switch, however the sound unit sustains the production of the tone to decay in similar to that in the continuous key depression when actuating the tone-sustaining switch. Thus, the tone-sustaining controller incorporated in the prior-art electronic keyboard instrument is operative to change the decay time period of a tone and is capable of imparting a tone-sustaining effect or a sostenuto effect to the tone produced in the prior-art electronic keyboard instrument.
However, a problem is encountered in the prior-art tone-sustaining unit in ignoring the actuation of the tone-sustaining switch upon successive key depressions. In detail, the latch circuit temporally stores the key state signal upon actuation of the tone-sustaining switch, however the latch circuit ignores the subsequent key state signal fed thereto during continuous actuation of the tone-sustaining switch because of lack of the timing signal. This results in rapid termination of the tone produced upon the subsequent key operation even if the tone-sustaining switch is actuated.